1. Field of the Invention
The present invention relates to handheld type four-cycle engines which are mainly used as a power source for machines for portable operation such as trimmers. More particularly, it relates to improvement of a four-cycle engine that includes an engine main body, the engine main body including a crankcase having a crank chamber, a cylinder block having a cylinder bore and a cylinder head having an intake port and an exhaust port; a crankshaft supported in the crankcase and housed inside the crank chamber; a piston fitted in the cylinder bore and connected to the crankshaft; an intake valve and an exhaust valve for opening and closing the intake port and exhaust port, the intake valve and exhaust valve being mounted in the cylinder head; a valve operation mechanism operable in association with the rotation of the crankshaft so as to open and close the intake valve and exhaust valve; and a power output or takeoff mechanism provided on one end of the crankshaft, the end projecting out of the engine main body.
2. Description of the Prior Art
Such a handheld type four-cycle engine is already known as disclosed in, for example, Japanese Patent Application Laid-open No. 10-288019.
Handheld type four-cycle engines are of course useful in terms of the prevention of environmental pollution as well as assuring the operators""s health since the exhaust gas is comparatively clean. However, since the structure thereof is more complicated than that of two-cycle engines, there is a drawback that it is difficult to reduce the weight thereof. Weight reduction is an important issue for improvements particularly in the operability of handheld four-cycle engines.
However, in the handheld type four-cycle engine disclosed in the above-mentioned patent publication, a valve operation mechanism for opening and closing intake and exhaust valves provided in the upper part ,of a cylinder head is of a type that includes pushrods and rocker arms, and a valve operation chamber for housing the pushrods, a camshaft for driving the pushrods, etc. is formed in a side wall of the engine main body; the size of the engine main body therefore inevitably increases thus making it difficult to reduce the weight of the engine.
The present invention has been carried out in view of the above-mentioned circumstances, and it is an object of the present invention to provide a lightweight handheld type four-cycle engine having good operability by making the engine main body compact.
In accordance with a first characteristic of the present invention in order to achieve the above-mentioned objective, there is proposed a handheld type four-cycle engine including an engine main body, the engine main body including a crankcase having a crank chamber, a cylinder block having a cylinder bore and a cylinder head having an intake port and an exhaust port; a crankshaft supported in the crankcase and housed inside the crank chamber; a piston fitted inside the cylinder bore and connected to the crankshaft; an intake valve and an exhaust valve for opening and closing the intake port and exhaust port, the intake valve and the exhaust valve being mounted in the cylinder head; a valve operation mechanism operable in association with the rotation of the crankshaft so as to open and close the intake valve and the exhaust valve; and a power output mechanism provided on one end of the crankshaft projecting out of the engine main body, wherein the valve operation mechanism includes a camshaft rotatably supported in the cylinder head so as to open and close the intake valve and the exhaust valve, and a dry type timing transmission placed outside the engine main body on the side opposite the power output mechanism and providing association between the crankshaft and the camshaft.
The above-mentioned power output mechanism corresponds to the centrifugal clutch described in the embodiments below.
In accordance with the above-mentioned first characteristic, since the timing transmission and the power output mechanism are mounted on either side of the cylinder head on the two ends of the crankshaft, the weight balance at the two ends of the crankshaft is improved, the centre of gravity of the engine can be made as close to the central part of the crankshaft as possible, which, together with the reduced weight, can enhance the operability of the engine. Furthermore, since the loads arising from the timing transmission and the drive shaft separately work on the two ends of the crankshaft during operation of the engine so avoiding the load on the crankshaft and its bearings from being localised, the durability thereof can be enhanced.
In accordance with a second characteristic of the present invention, in addition to the above-mentioned first characteristic, there is proposed a handheld type four-cycle engine wherein the timing transmission is made as a dry type and is separate from the crank chamber.
In accordance with the above-mentioned second characteristic, since it is unnecessary to provide the side wall of the engine main body with a special chamber for housing the timing transmission, the engine main body can be made thinner and more compact thus achieving a large reduction in the weight of the entire engine.
In accordance with a third characteristic of the present invention, in addition to the above-mentioned first or second characteristic, there is proposed a handheld type four-cycle engine wherein a flywheel is mounted on the crankshaft between the engine main body and the power output mechanism, the flywheel including cooling vanes for sending cooling air to the engine main body and having a diameter larger than that of the power output mechanism.
In accordance with the above-mentioned third characteristic, the cooling air can be supplied appropriately to the engine main body, without obstruction from the power output mechanism, by rotation of the cooling vanes while minimising any increase in the size of the engine due to the flywheel, and the cooling performance thereof can be enhanced.
In accordance with a fourth characteristic of the present invention, in addition to the above-mentioned first or second characteristics there is proposed a handheld type four-cycle engine wherein an oil tank for storing a lubricating oil for lubricating the inside of the engine main body is placed outside the timing transmission so as to adjoin it and is supported on the engine main body.
In accordance with the above-mentioned fourth characteristic, since the oil tank covers at least one part of the timing transmission, the transmission can be protected. Moreover, since the oil tank and the flywheel are positioned opposite to each other, the centre of gravity of the engine can be made as close to the central part of the crankshaft as possible and the operability of the engine can be further enhanced.
In accordance with a fifth characteristic of the present invention, in addition to the above-mentioned first characteristic, there is proposed a handheld type four-cycle engine wherein the valve operation mechanism includes the timing transmission placed outside the engine main body and linked to one end of the crankshaft and a cam system for transmitting the rotational force of the driven side of the timing transmission to the intake and exhaust valves for opening and closing forces, a first valve mechanism chamber housing the timing transmission is provided integrally with an oil tank that is placed outside of the engine main body on the same side as the timing transmission, a second valve mechanism chamber housing at least one part of the cam system is formed in the cylinder head, and a pair of oil slingers for stirring and scattering the oil stored in the oil tank in order to generate an oil mist that is to be supplied to the second valve operation chamber and the crank chamber are fixed to the crankshaft so that the timing transmission is interposed between the pair of the slingers.
In accordance with the above-mentioned fifth characteristic, since the oil tank is placed on one side outside the engine main body, the total height of the engine can be greatly reduced. Moreover, since the first valve operation chamber housing the timing transmission is provided integrally with the oil tank, one part of the timing transmission is housed in the oil tank so making the engine more compact.
Furthermore, since the lubrication system of the valve operation mechanism is divided into two parts, that is, a part for lubricating the timing transmission inside the first valve operation chamber with the oil scattered inside the oil tank, and a part for lubricating the cam system inside the second valve operation chamber with the oil mist generated inside the oil tank, the load put on each part of the lubrication system is lessened and the entire valve operation mechanism can be lubricated thoroughly.
Moreover, the pair of the oil slingers are fixed to the crankshaft with the timing transmission is placed therebetween, the oil stored inside the oil tank can be stirred and scattered without obstruction from the timing transmission regardless of the operational position of the engine and the oil mist can be generated effectively.
Furthermore, in accordance with a sixth characteristic of the present invention, in addition to the above-mentioned fifth characteristic, there is proposed a handheld type four-cycle engine wherein a through hole through which the oil mist generated in the oil tank is supplied to the crank chamber is provided in the crankshaft, and an open end of the through hole in the oil tank is positioned between the timing transmission and an oil slinger.
In accordance with the above-mentioned sixth characteristic, the open end of the through hole of the crankshaft can be positioned in the central area of the oil tank or in the vicinity thereof without obstruction from the timing transmission or the oil singers, and it is possible to prevent the oil stored inside the oil tank from entering the through hole directly.
Furthermore, in accordance with a seventh characteristic of the present invention, in addition to the above-mentioned fifth characteristic, there is proposed a handheld type four-cycle engine wherein the oil tank for storing lubricating oil and the timing transmission of the valve operation mechanism are placed on one side of the engine main body, the timing transmission extending into the oil tank, a belt guide tube housing the timing transmission is provided integrally with the oil tank, and the open end of the belt guide tube inside the oil tank projects towards the central part of the oil tank so that the open end is above the liquid level of the stored oil regardless of whether the engine is upside down or laid on its side.
In accordance with the above-mentioned seventh characteristic, the total height of the engine can be reduced, at the same time any increase in the width of the engine can be minimised, and the engine can therefore be made more compact. Moreover, since the open end inside the oil tank of the belt guide tube housing the timing transmission is always above the liquid level of the stored oil even when the engine is upside down or laid on its side, the stored oil is prevented from flowing towards the timing transmission, oversupply of oil to the timing transmission can be prevented and at the same time the amount of oil stored in the oil tank can be maintained at a predetermined level.
Furthermore, in accordance with an eighth characteristic of the present invention, in addition to the above-mentioned fifth characteristic, there is proposed a handheld type four-cycle engine wherein the oil tank, an end of the crankshaft extending into the oil tank, and the timing transmission of the valve operation mechanism linked to the crankshaft inside the oil tank are placed outside the engine main body on the side opposite to the power output mechanism, and the timing transmission is lubricated by the oil inside the oil tank.
In accordance with the above-mentioned eighth characteristic, it is unnecessary to provide a special chamber for housing the timing transmission in the side wall itself of the engine main body, the total height of the engine can be reduced due to the sideways arrangement of the oil tank, the side wall of the engine main body can thus be made thinner and more compact, and the weight of the entire engine can be greatly reduced. Moreover, the weight balance at the two ends of the crankshaft is improved by placing the power output mechanism on one side of the engine main body and the timing transmission and the oil tank on the other side, the centre of gravity of the engine can be made as close to the central part of the crankshaft as possible, which, together with the reduced weight, can enhance the operability of the engine.
Moreover, since the loads arising from the timing transmission and the power output mechanism during operation of the engine separately work on the two ends of the crankshaft so avoiding the load on the crankshaft and its bearings from being localised, the durability thereof can be enhanced.
Furthermore, since the timing transmission is lubricated directly with oil inside the oil tank, the lubrication system can be simplified.
In accordance with a ninth characteristic of the present invention, in addition to the above-mentioned eighth characteristic, there is proposed a handheld type four-cycle engine wherein a cooling fan is fixed to the crankshaft between the engine main body and the power output mechanism, the cooling fan having a diameter larger than that of the power output mechanism.
In accordance with the above-mentioned ninth characteristic, any increase in size of the engine can be minimised while enhancing the air supply performance of the cooling fan.
In accordance with a tenth characteristic of the present invention, in addition to the above-mentioned eighth characteristic, there is proposed a handheld type four-cycle engine wherein the cam system for transmitting the rotation of the driven side of the timing transmission to the intake valve and the exhaust valve for opening and closing forces is placed in the valve operation chamber provided in the cylinder head, and oil mist generation means for generating an oil mist inside the oil tank is linked to the crankshaft, the oil mist being supplied to the valve operation chamber.
In accordance with the above-mentioned tenth characteristic, since the lubrication system of the valve operation mechanism is divided into two part, that is, a part for lubricating the timing transmission with oil inside the oil tank, and a part for lubricating the cam system with oil mist generated inside the oil tank, the load put on each part of the lubrication system is lessened and the entire valve operation mechanism can be lubricated thoroughly.
Furthermore, in accordance with an eleventh characteristic of the present invention, in addition to the above-mentioned fifth characteristic, there is proposed a handheld type four-cycle engine the timing transmission of the valve operation mechanism is constructed as a wrap-around type having a wrap-around member, the drive side of the wrap-around member extending into the oil tank, oil mist generation means for generating an oil mist for lubricating the timing transmission by scattering oil stored inside the oil tank is provided in the oil tank, and an oil droplet guide wall is provided so as to project out of the inner wall of the oil tank, the oil droplet guide wall guiding and dripping the attached oil droplets onto the part of the timing transmission extending into the oil tank when the engine is laid on its side.
In accordance with the above-mentioned eleventh characteristic, when the engine is operated in a laid-sideways state, the oil mist attached to the oil droplet guide wall turns into oil droplets, the droplets then fall down onto the wrap-around member on the drive side of the timing transmission and, in particular, when the upper part of the wrap-around member moves from the drive side to the driven side, the above-mentioned oil droplets can be carried by the wrap-around member to the driven side with hardly any influence from centrifugal force and the driven side can be lubricated reliably.
The above-mentioned wrap round member corresponds to the timing belt 25, 125, 225 in the embodiments of the present invention described below.
Furthermore, in accordance with a twelfth characteristic of the present invention, in addition to the above-mentioned first characteristic, there is proposed a handheld type four-cycle engine wherein the valve operation mechanism is provided over an oil tank placed outside the engine main body and storing a lubricating oil, a first valve operation chamber formed so as to extend upwards from the oil tank, and a second valve operation chamber formed in the cylinder head; the oil tank and the crank chamber are communicated with each other by means of a through hole; the crank chamber and the second valve operation chamber are communicated with each other by means of an oil feed pipe provided outside the engine main body; the second valve operation chamber and the oil tank are communicated with each other by means of an oil return passage; the oil tank includes oil mist generation means for generating an oil mist by stirring and scattering the stored oil; and transfer means for transferring the oil mist inside the oil tank to the oil feed pipe via the crank chamber is connected to the oil feed pipe so that the valve operation mechanism inside the first valve operation chamber is lubricated with the oil scattered inside the oil tank; and the valve operation mechanism inside the second valve operation chamber is lubricated with oil mist transferred from the oil feed pipe to the second valve operation chamber.
In accordance with the twelfth characteristic of the present invention, since the oil feed pipe is placed outside the engine main body, it is possible to make the side wall of the engine main body thinner regardless of the presence of the pipe, the engine main body can be made more compact and the weight of the entire engine can thus be reduced. Moreover, since the oil feed pipe outside the engine main body easily radiates heat, cooling of the oil mist passing through the pipe can be improved.
Since the lubrication system of the valve operation mechanism is divided into two parts, that is, a part for lubricating the valve operation mechanism inside the oil tank and the first valve operation chamber with the oil scattered inside the oil tank, and a system for lubricating the valve operation mechanism inside the second valve operation chamber with the oil mist transferred to the second valve operation chamber, the load put on each part of the lubrication system is lessened and the entire valve operation mechanism can be lubricated thoroughly.
Moreover, each part of the engine can be lubricated reliably regardless of the operational position of the engine by the use of oil droplets and oil mist.
Furthermore, in accordance with a thirteenth characteristic of the present invention, in addition to the above-mentioned twelfth characteristic, there is proposed a handheld type four-cycle engine wherein the transfer means includes valve means that closes the oil feed pipe when the pressure of the crank chamber is negative and opens the pipe when the pressure is positive.
In accordance with the thirteenth characteristic of the present invention, it is unnecessary to employ a special oil pump for circulating the oil mist, and the structure can be simplified.
Furthermore, in accordance with a fourteenth characteristic of the present invention, in addition to the above-mentioned twelfth or thirteenth characteristic, there is proposed a handheld type four-cycle engine wherein the oil feed pipe and the oil return passage are connected to each other via a bypass.
In accordance with the above-mentioned fourteenth characteristic, the amount of oil mist supplied to the second valve operation chamber from the oil feed pipe can be controlled by appropriately selecting the flow resistance of the bypass.
In accordance with a fifteenth characteristic of the present invention, in addition to the above-mentioned first characteristic, there is proposed a handheld type four-cycle engine wherein the valve operation mechanism includes the timing transmission which has a rotating drive member linked to the crankshaft and a cam system for transmitting the rotational force of a rotating driven member of the timing transmission to the intake valve and exhaust valve for opening and closing forces; a first valve operation chamber and an oil tank are provided on one side of the engine main body, the first valve operation chamber housing the timing transmission, the oil tank including oil mist generation means for generating an oil mist from stored oil, and the lower end of the first valve operation chamber opening inside the oil tank; a second valve operation chamber housing the cam system is provided in the upper part of the engine main body so as to be in line with the first valve operation chamber; a first lubrication system includes first and second oil passages placed alongside each other and providing communication between the oil tank and the crank chamber, and first oil feed means for circulating the oil mist generated inside the oil tank from the oil tank via the first oil passage, the crank chamber, and the second oil passage, back to the oil tank; and a second lubrication system includes a third oil passage providing communication between the first valve operation chamber and the second valve operation chamber, a fourth oil passage providing communication between the second valve operation chamber and the crank chamber, the second oil passage, and second oil feed means for circulating the oil mist generated inside the oil tank from the oil tank via the first valve operation chamber, the third oil passage, the second valve operation chamber, the fourth oil passage, the crank chamber, and the second oil passage, back to the oil tank. The rotating drive member and the rotating driven member correspond to the drive pulley 223 and the driven pulley 224 of the third embodiment of the present invention described below, and the oil mist generation means corresponds to the oil slingers 256a and 256b. 
In accordance with the fifteenth characteristic, since the surroundings of the crank shaft are lubricated by the first and second lubrication systems, and the timing transmission and the cam system of the valve operation system are lubricated by the second lubrication system, the circumference of the crankshaft which is subjected to a comparatively high load can be lubricated adequately, at the same time it is possible to prevent excessive lubrication of the valve operation mechanism which is subjected to a comparatively low load, the amount of oil mist circulated can be minimised, the amount of oil stored in the oil tank can be reduced, and not only the oil tank but also the entire engine can be made more compact and lighter.
In accordance with a sixteenth characteristic of the present invention, in addition to the above-mentioned fifteenth characteristic, there is proposed a handheld type four-cycle engine wherein the first oil feed means includes a first one-way valve provided in the second oil passage, closing when the pressure of the crank chamber decreases and opening when the pressure increases, and the second oil feed means includes a second one-way valve provided in the third oil passage, closing when the pressure of the crank chamber decreases and opening when the pressure increases.
In accordance with the sixteenth characteristic, the oil mist inside the oil tank can be circulated by utilising the pressure pulsations within the crank chamber and the one-way transfer functions of the first and second one-way valves, it is therefore unnecessary to employ a special oil pump for circulation of the oil mist and the structure can thus be simplified.